The invention relates to a switch device and more particularly to a switch device to be responsive to given positions of a piston and cylinder unit, comprising a switch which is to be mounted externally on the barrel of the cylinder and piston unit and which has two terminals adapted to be connected with signal conductors, for example conductors in the form of flexible wires.
Such switch devices are employed to indicate certain positions of moving components or in order to cause certain switching operations to take place in response to given settings of such components. Most frequently such switch devices are used in conjunction with piston and cylinder units in order to respond to certain positions of the piston in such a unit. For instance, they may be used to indicate when the piston of the unit is in its end positions, so as to cause a reversal in the direction of motion of the piston when its reaches certain positions or to cause one of a variety of other possible functions. The switch used is preferably in the form of a reed contact switch which has two spaced switching contacts and is arranged externally on the outer peripheral surface of the cylinder barrel. The actuation of the switch is caused in a contactless manner by the piston itself, which for this purpose carries a permanent magnet which operates the two switching contacts as the piston moves by owing to the action of the magnetic field.
Since such switches are relatively sensitive to external effects it is recommended to accommodate them in a housing. However this means that the switch unit then has relatively large dimensions so that a piston and cylinder unit fitted with such a switch device has a large radial dimension for part of its length; this means that the switch device is only rarely suitable when there is a shortage of space. However it is in such applications, as for instance in industrial robots, that the use of such switches is required. A further point is that the arrangement may often not be designed to have a small enough distance between the switch and the cylinder with the result that the magnetic field does not reliably operate the switch.